Add support for using __bool__ method literal value in union narrowing in if statements

[kprzybyla]

This adds support for union narrowing in if statements when condition value has defined literal annotations in __bool__ method. Value is narrowed based on the __bool__ method return annotation and this works even if multiple instances defines the same literal value for __bool__ method return type.

This PR also works well with #9288 and makes below example to work as expected:

class A:
    def __bool__(self) -> Literal[True]: ...

class B:
    def __bool__(self) -> Literal[False]: ...

def get_thing() -> Union[A, B]: ...

if x := get_thing():
    reveal_type()  # Revealed type is '__main__.A'
else:
    reveal_type()  # Revealed type is '__main__.B'

Partially fixes #9220

[JukkaL]

Thanks for the PR! The code is nice and clean, and there is good test coverage.